i8 



THE PLANT CELL 



cytoplasm, vacuoles, and nuclear cavity. The powers of osmosis 

 and diffusion are at work here, essentially as we find them m 

 artificial cells and through artificial and Hfeless membranes; but 

 the chemical work done by the living protoplast makes and 

 keeps up the conditions necessary to osmosis and diffusion, and 

 the plasma membranes modify, and to a certain extent regulate, 

 the osmotic and diffusion exchange of materials into and from 

 the body of the protoplast. 



Fig. 9. — I and 2, nuclei from the tapetal Plasmodium of the sporangium of Botrychium 

 Virginianum, i, in the usual form, and 2, flattened while entering a crevice between spore- 

 groups; 3, cell from Nitella showing rotatioh of the cytoplasm as indicated by the arrows; 

 4, cell from stamen-hair of Tradescantia, showing circulation of the cytoplasm as indicated 

 by the arrows. ■ 



(b) Construction. — The protoplast builds complex substances 

 from simpler ones, as when sugar, starch, oil, and proteid foods 

 are built from carbon dioxide, water, and salts of nitrogen and 

 sulphur; and when the foods are assimilated to the substance 

 of the protoplast itself. 



(c) Destruction. — In respiration the protoplast breaks down, 

 a part of its own substance, and probably also reserve foods, into 

 simpler compounds; and the self-destruction of the protoplast 

 appears to take place in the production of secretions, such as 

 cell-wall and digestive ferments. 



(d) Excretion. — Frequently substances that are of no use are 

 excluded from the protoplast by inclosure in an insoluble form 



